Pyomyositis (PM) is an infectious disease of the skeletal muscle with a wide range of symptoms such as pain, fever or swelling, and is predominantly found in the tropics. In recent years PM has increasingly been diagnosed in Europe and in the U.S. Our objective is to describe the ultrasound and MRI features of PM in children. A retrospective analysis of 12 children with PM (2 girls and 10 boys; age range 1-13 years) admitted to our hospital between 1998 and 2002 was carried out. All children had a US exam and 8 children underwent MRI. Children with osteomyelitis and accompanying myositis were excluded from this study. In all patients who had MRI ( n=8) the infected muscles were found to have the following features: hyperintensity on the T2-weighted images, diffuse borders and contrast enhancement. In the pelvis ( n=4), only one PM could be detected with US, in the other 3 cases only US of the hip joint was performed based on the clinical symptoms. In the extremities ( n=8) US always revealed an altered echogenicity of the affected muscles as well as fluid collection in 5 cases. Both US and MRI reveal characteristic changes of the PM. Ultrasound should be the first imaging modality in the extremities. In the pelvis MRI is the imaging modality of choice. The MRI is needed to differentiate pyomyositis from osteomyelitis.